Last week we tried to design a blood pressure monitor (BPM), now let's see some applications!

The Flexis QE128 microcontrollers form the connection point on the Freescale Controller Continuum, where complimentary families of 8-bit S08 and 32-bit ColdFire V1 microcontrollers share a common set of peripherals and development tools to deliver the ultimate in migration flexibility. The Flexis QE128 microcontrollers enable compatibility between low-end and high-end products. This gives designers and developers the ability to design one software and hardware platform and add and detract hardware and software to accommodate the different features in various products.

The figure shows the block diagram for the high-end blood pressure monitor demo. Lower-end designs can use the same hardware design and printed circuit board, but not populate areas that are not used in the system. For example, a low-end application may not have the ability to have connectivity on it; therefore, the bill of materials for the design may not include the hardware associated with connectivity.

The software throughout the platform is developed to be non-blocking; therefore, the MCU has the ability to attend several different tasks without the need for an operating system. Every module works as an independent state machine that is automatically updated every time the MCU runs through the code, or as an interrupt-based process. Code written in this format enables the MCU to always return to the main loop and continue operating even when one task stops working correctly.

Heartbeat Detection
While deflating a cuff that is attached to a person’s arm, a you can see slight variations in the overall pressure on the cuff. This variation in the pressure on the cuff is actually due to the pressure change from blood circulation. This variation is amplified through a high-pass filter designed at 1 Hz, and set to an offset. This new signal is the heartbeat signal.

Heartbeat Signal

Heartbeat Over Time

Systolic and Diastolic Measurements Using HCS08

 Using the heartbeat detection as explained, a simple oscillometric method is used to determine systolic blood pressure (SBP) and diastolic blood pressure (DBP). The simplified measurement is based on the idea that the amplitude of the heartbeat signal changes as the cuff is inflated over the SBP. While the cuff is deflated, the amplitude of the heartbeat signal grows as the cuff pressure passes the systolic pressure on the patient. As the cuff pressure is further reduced, the pulsations increase in amplitude, until it reaches a maximum pulse known as the mean arterial pressure (MAP), and then reduces rapidly until the diastolic pressure is reached.

Heartbeat Versus Diastolic Pressure

Systolic and Diastolic Measurements Using Coldfire V1
If using the simple oscillometric method, the cuff on a patient’s arm has to be inflated higher than the systolic pressure of the patient. The problem here is that the system does not know the systolic pressure of the patient, and so, it over inflates the cuff to make sure that it is able to find the systolic pressure. This is uncomfortable for patient. Using the Coldfire V1, the system can employ a reverse oscillometric method. In this method, the 32-bit core can filter out system noise that is added by the motors while inflating the cuff.

Read the Italian version: Misurazione della pressione sanguigna tramite l’uso della famiglia Flexis QE128 di Freescale

Source: Freescale Blood Pressure Monitor

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